1. Field of the Invention
The present invention generally relates to television tuner systems and, more particularly, to a circuit layout (arrangement) of a television tuner system.
2. Description of the Related Art
FIG. 3 is a plan view illustrating a circuit layout of a known television tuner system. Referring to FIG. 3, a metallic frame 51 has four side plates 51a, 51b, 51c, and 51d, and a connector 52 for receiving television signals is attached to the side plate 51d of the frame 51. A printed board 53 is disposed within the frame 51. Formed on the printed board 53 are a tuner disposed in a region 53a near the connector 52 and an intermediate frequency circuit in a region 53b away from the connector 52. Ground conductors 54 and 55, which are connected to the frame 51, are formed in parallel to each other at the boundary between the regions 53a and 53b. The ground conductor 54 is formed at the periphery of the region 53a of the printed board 53 and is connected to the frame 51 with a ground conductor 56 connected to the side plate 51a therebetween. The ground conductor 55 is formed at the periphery of the region 53b of the printed board 53 and is connected to the frame 51 with a ground conductor 57 connected to the side plate 51a therebetween.
In the region 53a where the tuner is formed, there are disposed not only a high frequency amplifier and a tuning circuit, both of which are not shown, but also a mixer, a local oscillator, and an intermediate frequency amplifier. The mixer, the local oscillator, and the intermediate frequency amplifier are integrated into an integrated circuit (IC) component 58 within a single package. A tuning circuit connected to the local oscillator within the IC component 58 is disposed in a region 53c adjacent to the ground conductor 54. The IC component 58 is placed in proximity with the region 53c. 
With the above arrangement, television signals input into the connector 52 are amplified in the high frequency amplifier (not shown), and a television signal RF of a desired channel is selected in the tuning circuit (not shown). The television signal RF is then input into the mixer within the IC component 58. In the mixer, the television signal RF is mixed with an oscillation signal output from the local oscillator and is converted into an intermediate frequency signal. The intermediate frequency signal is temporarily extracted from the IC component 58 and input into an intermediate frequency tuning circuit 59 formed on the printed board 53, and again returns to the intermediate frequency amplifier formed within the IC component 58. The intermediate frequency signal is amplified in the intermediate frequency amplifier, and the amplified signal IF is then output via a conductor pattern 60 formed on the printed board 53 to the intermediate frequency circuit disposed in the region 53b. 
A surface acoustic wave (SAW) filter 61 and a detector 62 are provided for the intermediate frequency circuit in the region 53b. The SAW filter 61 is formed generally in a rectangular prism shape and has a plurality of terminals, such as an input terminal 61a, a ground terminal 61b, and output terminals 61c and 61c, which are sequentially aligned and project in the same direction from one side of the SAW filter 61. The SAW filter 61 is placed in proximity with the ground conductor 55 in such a manner that the terminals 61a, 61b, 61c and 61c are aligned parallel to the ground conductor 55 and that the output terminals 61c and 61c are placed near the ground conductor 57. In this state, the input terminal 61a is connected to the conductor pattern 60, and the ground terminal 61b is connected to a ground conductor portion 55a projecting from the ground conductor 55.
The detector 62 is also formed generally in a rectangular prism shape as an IC component. Intermediate frequency signal input terminals 62a and 62a, and detector output terminals 62b and 62c project from the respective two sides of the detector 62. The detector 62 and the SAW filter 61 are placed side by side in such a manner that the intermediate frequency signal input terminals 62a and 62a face the SAW filter 61. The output terminals 61c and 61c of the SAW filter 61 are respectively connected to the intermediate frequency signal input terminals 62a and 62a of the detector 62 with conductor patterns (not shown). The detector output terminals 62b and 62c are respectively connected via conductor patterns 63 and 64 to terminals 65 and 66 attached to the side plate 51c of the frame 51.
An intermediate frequency signal input into the input terminal 61a of the SAW filter 61 is provided with predetermined selectivity characteristics in the SAW filter 61, and the balanced signals are output from the output terminals 61c and 61c. The balanced signals are then input into the intermediate frequency signal input terminals 62a and 62a of the detector 62 and are detected by the detector 62, thereby extracting a video signal V and a sound signal S.
However, the above conventional television tuner system presents the following problems.
The SAW filter 61 is located on the printed board 53 in such a manner that the terminals 61a, 61b, 61c and 61c are aligned substantially parallel to the two ground conductors 54 and 55 formed at the boundary of the two regions 53a and 53b of the printed board 53. Accordingly, the output terminals 61c and 61c, as well as the input terminal 61a, of the SAW filter 61 are placed in proximity with the local oscillator (which is integrated within the IC component 58) disposed in the region 53a. Thus, an oscillation signal output from the local oscillator may be accidentally input into the output terminals 61c and 61c of the SAW filter 61, and the level of the signal is accordingly increased. The oscillation signal is then input into the detector 62. Since the detector 62 is provided with a voltage controlled oscillator for synchronous detection, an unwanted interruption signal (beat signal) is generated due to the local oscillation signal from the detector 62 and the oscillation signal from the voltage controlled oscillator.
The ground terminal 61b of the SAW filter 61 is connected to the frame 51 with the ground conductor portion 55a therebetween, thereby increasing the distance from the ground terminal 61b to the frame 51. According, the grounding effect of the ground terminal 61b is impaired. Thus, the selectivity characteristics of the SAW filter 61 are lowered, thereby failing to sufficiently attenuate the band in an adjacent channel.
Accordingly, it is an object of the present invention to inhibit a local oscillation signal from being accidentally input into output terminals of a SAW filter, which would otherwise cause the generation of interruption signals, and also to maintain the performance of the SAW filter by preserving its selectivity characteristics.
In order to achieve the above object, according to the present invention, there is provided a television tuner system including a printed board disposed in a metallic frame having a side plate. A tuner has a mixer and a local oscillator and converts a received television signal into an intermediate frequency signal. An intermediate frequency circuit has at least a SAW filter and detects the intermediate frequency signal output from the tuner. The tuner and the intermediate frequency circuit are arranged adjacent to each other on the printed board. A first ground conductor is connected to the frame and is disposed on the printed board between the tuner and the intermediate frequency circuit in such a manner that it is disposed substantially perpendicular to the side plate. A second ground conductor is connected to the frame and is disposed along the side plate at a peripheral edge of the printed board corresponding to the position of the intermediate frequency circuit. The SAW filter has at least an input terminal and an output terminal, which are aligned in a row and project in the same direction. The SAW filter is placed along the second ground conductor in such a manner that the terminals are disposed substantially parallel to the side plate of the frame. The local oscillator is disposed substantially on a line extending from the arrangement of the terminals, so that a distance between the output terminal and the local oscillator is greater than a distance between the input terminal and the local oscillator.
According to the aforementioned television tuner system, the SAW filter may include a ground terminal between the input terminal and the output terminal. A third ground conductor may be provided on the printed board in such a manner that it is connected to the second ground conductor and extends to an inward portion of the frame. Thus, the ground terminal may be connected to the third ground conductor.
Further, the intermediate frequency circuit may include a detector for detecting the intermediate frequency signal output from the SAW filter. A fourth ground conductor, which is connected to the frame, may be formed at a peripheral edge of the printed board corresponding to the position of the intermediate frequency circuit along a plate adjacent to the side plate. Thus, the first ground conductor, the second ground conductor, and the fourth ground conductor may be formed in an angular U-shape, and the detector and the SAW filter may be, as a whole, placed side by side.
Further, the detector may include an intermediate frequency signal input terminal and a detector output terminal, which project in opposite directions. A fifth ground conductor may be disposed on the printed board in such a manner that it is connected to the fourth ground conductor and extends to an inward portion of the frame. The detector and the SAW filter may be placed side by side on the printed board in such a manner that the intermediate frequency signal input terminal faces the SAW filter. Thus, the fifth ground conductor may be positioned between the intermediate frequency signal input terminal and the detector output terminal.
In addition, the second ground conductor may be connected to the frame at a position corresponding to a portion of the third ground conductor projecting from the second ground conductor. The fourth ground conductor may be connected to the frame at a position corresponding to a portion of the fifth ground conductor projecting from the fourth ground conductor.